Nucleation Probability Distributions of Methane–Propane Mixed Gas

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Nucleation Probability Distributions of Methane−Propane Mixed Gas Hydrates in Salt Solutions and Urea Barbara Sowa,†,‡ Xue Hua Zhang,§ Karen A. Kozielski,∥ Patrick G. Hartley,† Dave E. Dunstan,‡ and Nobuo Maeda*,† †

CSIRO Materials Science & Engineering and ∥CSIRO Earth Science and Resource Engineering, Ian Wark Laboratory, Ian Wark Laboratory, Bayview Avenue, Clayton, Victoria 3168, Australia ‡ Department of Chemical and Biomolecular Engineering, School of Engineering, University of Melbourne, Melbourne, Victoria 3010, Australia § School of Civil, Environmental and Chemical Engineering, RMIT University, Melbourne, Victoria 3001, Australia S Supporting Information *

ABSTRACT: We studied nucleation probability distributions of a model natural gas (a mixture of 90% methane and 10% propane; C1/C3) hydrate in aqueous solutions of salts using the combinations of seven anions (F−, Cl−, Br−, I−, SCN−, NO3−, SO42−) and seven cations (NH4+, K+, Na+, Mg2+, Ca2+, Mn2+, Al3+) at a broad range of concentrations. A high pressure automated lag time apparatus (HP-ALTA) was used for the study. HP-ALTA can apply a large number (>100) of linear cooling ramps under isobaric conditions to the sample and construct nucleation probability distributions of C1/C3 mixed gas hydrate for each sample. We found that (1) ions can promote or inhibit gas hydrate formation at concentrations below 1 M, (2) this promotion or inhibition effect of salts did not depend on the valency of the ions involved for the range of salts studied, (3) the width of the nucleation probability distributions (stochasticity) of gas hydrate decreased in the presence of ions, (4) this decrease in the distribution width was largely independent of the type of ions involved or the concentration of the salt below 100 mM. We also extended the study to include the effects of urea and a quaternary ammonium salt (an antiagglomerate). The results showed that the formation of C1/C3 mixed gas hydrate was inhibited more strongly by urea than any of the inorganic salts at 1 M or above, suggesting that urea may serve as an effective and environmentally friendly hydrate inhibitor at high concentrations. The quaternary ammonium salt showed a modest inhibition effect at all ranges of concentrations. The study was extended to the formation of ice and model tetrahydrofuran hydrate, for comparison.

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INTRODUCTION

homopolymers and copolymers of vinyl caprolactam or poly(ester amides).8 The investigation of electrolytes in aqueous systems has always been an important research topic.9−12 There are numerous areas in chemical and petroleum engineering where the knowledge about the phase behavior of salt solutions is required for better understanding of problems, such as inhibition of gas hydrate formation in offshore/arctic drilling operations, flow assurance of oil and gas pipelines, among others.13 Many experimental14,15 and modeling16,17 studies have been reported about gas hydrate stability conditions in the presence of inorganic salts. Detailed knowledge about the distribution of ions in the vicinity of the air−aqueous interface is of great importance for understanding of gas hydrates. Our recent study18 showed that strong monovalent salt solutions may act as kinetic hydrate promoters or inhibitors at low concentrations ( K2SO4 > MnSO4 ∼ Al2(SO4)3 ∼ Na2SO4. Fourth, the higher the concentration (above 1 M), the stronger the thermodynamic inhibition effect. Magnesium chloride and ammonium chloride showed the highest subcooling, ΔT (the strongest inhibition), at the highest concentration studied (1 M and 3 M). Finally, it can be seen that not every salt studied showed strong thermodynamic inhibition effect at concentrations above 1 M. Surprisingly, most of the salts that were studied at higher concentrations (above 1 M), except for MgCl2, NH4Cl, and NH4NO3, had no significant inhibition effect between 1 M and 3 M. Our result suggests that higher concentrations than 3 M are needed to detect any thermodynamic inhibition effect for some of the salts. Given that the solubility of some salts are lower than 3 M, some salts may not act at a THI at all. Quaternary Ammonium Salt and Urea. Figure 3 also showed that the quaternary ammonium salt, tetra-n-butylammonium bromide ((C4H9)4NBr, TBAB) consistently inhibited the formation of C1/C3 gas hydrate at all concentrations. In E

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Table 3. Effect of Different Combinations of Ions on ΔT of C1/C3 Mixed Gas Hydrates in Dilute Aqueous Solutions at Pressure of 7 MPaa

a The classification of different combinations of ions is based on the kosmotropic (K) and chaotropic (CH) properties of each ion. The promotion effect (green) is denoted by “+”, the inhibition effect (red) is denoted by “−”, and no effect (gray) is denoted by “0”. Data include results from the current study (Figure 3) and a previous paper.18

Formation Temperature Distributions of THF Model Hydrate and Ice in Salt Solutions. We extended our studies to model THF hydrate (Figure SI; Supporting Information) and ice (Figure S2; Supporting Information) systems, for comparison. The kinetic promotion/inhibition effect of salts on C1/C3 mixed gas hydrate was not always correlated with that in the model THF hydrate or the ice systems. Table 2 presents the summary of the effect of salts on C1/C3 mixed gas hydrates, model THF hydrate, and ice nucleation probability distributions. Only a few combinations showed a correlation: (1) calcium chloride, aluminum chloride, ammonium iodide (promoters of C1/C3 mixed gas hydrates and THF model hydrate); (2) sodium thiocyanate, ammonium nitrate and urea (inhibitors of C1/C3 mixed gas hydrates and THF model hydrate). In many cases, a promotion effect of a salt on C1/C3 mixed gas hydrate was correlated to a neutral or a promotion effect on THF model hydrate. However, (NH4)2SO4 exhibited a promotion effect on C1/C3 mixed gas hydrate and an inhibition effect on THF model hydrate. Conversely, an inhibition effect of a salt on C1/C3 mixed gas hydrate was often correlated with an inhibition or a neutral effect on THF model hydrate. A neutral effect of a salt on C1/C3 mixed gas hydrate formation corresponded to a promotion or a neutral effect on THF hydrate. No clear correlation could be seen between the effect of salts on ice formation and C1/C3 mixed gas hydrate formation or THF model hydrate formation. In general, salts had a weak (neutral) effect on ice formation at a concentration below 1 M.

ions, (3) the width of the nucleation probability distribution of gas hydrate formation decreased due to the presence of ions for all salts, (4) this decrease in the distribution width was largely independent of the salt concentrations below 100 mM, (5) the nucleation of C1/C3 mixed gas hydrate was inhibited more strongly by urea than by any of the inorganic salts at 1 M or above, (6) urea exhibited no significant inhibition effect at 10 mM or below, (7) the quaternary ammonium salt (TBAB) modestly hindered C1/C3 mixed gas hydrate formation at all range of concentrations, (8) any correlation between the C1/ C3 mixed gas hydrate system and the ice or the THF hydrate system was limited at best. We previously proposed the so-called two-step mechanism of gas hydrate formation: (1) gas dissolution and then (2) nucleation.24,36 We note that this is different from what Jacobson et al.37 and Vatamanu et al. had proposed.38 In the two-step mechanism the nucleation of gas hydrates can take place only when sufficient amounts of gas molecules are present in the aqueous phase. The strength of the hydrogen bonds at the aqueous surface affects the mass transfer/dissolution rate of gases into the aqueous phase which affect the buildup of supersaturation (driving force) for gas hydrate formation. We assumed that the presence of salts and urea might influence the structure of water at the interface and in the bulk as well as gas solubility. To understand the complex effects of the salts and urea on gas hydrate formation, it is important to investigate the combined effects of ions on gas dissolution and nucleation. Figure 4 phenomenologically classifies the effects of the salts on C1/C3 mixed gas hydrate formation into three groups: promotion (green), neutral (black), and inhibition (red). Figure 4 contains both the data from Figure 3 and from our previous study.18 We use Figure 4 as the basis for further analyses. We start with determining the effect of individual ions from Figure 4 using the following rule; if an ion appears in all three groups (green, black, red) then we consider that ion to have no promotion or inhibition effect. If an ion appears in two groups, e.g., red and black, then we consider that ion to have an inhibition effect. Likewise, if an ion appears in green and black, then we consider that ion to have a promotion effect.

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DISCUSSION Summary of the Results. We presented a detailed study on the effect of salts and urea on the formation of C1/C3 mixed gas hydrate, model THF hydrate, and ice. We used 16 inorganic salt solutions, a quaternary ammonium salt and urea at a broad range of concentration between 10−5 M and 3 M. It was found that (1) ions can promote or inhibit gas hydrate formation at low concentrations, depending on the specific ion combinations, (2) the promotion and the inhibition effect of ions at low concentrations did not depend on the valency of the F

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Effect on Gas Dissolution (Salting In/Salting Out Effect). One way that salts can inhibit the nucleation of gas hydrates is through a reduction in the solubility of guest gas in water. Although the available data in the literature on the solubility of methane or propane in electrolytes are limited, we found some correlation between the literature data and our results. It was reported that the surface tension of aqueous solutions increased with the electrolyte concentration in the order: NaF > NaCl > LiCl > KCl > NaBr > KBr > NaI > KI.10,39,40 The salting-out of a hydrophobic guest gas by a salt, and concomitant reduction in gas solubility, has an impact on the nucleation of gas hydrates. Such an impact will depend on whether the ions are in the vicinity of the aqueous−guest interface or not, because the nucleation of gas hydrate only occurs at the guest−aqueous interface where the solubility of gas is the highest. A positive or a negative surface tension slope of a salt solution as a function of the salt concentration is related to the negative or positive adsorption of ions to the interface, respectively. We thus expect (1) a positively adsorbing salting-out salt to have an inhibition effect, (2) a positively adsorbing salting-in salt to have a promotion effect, (3) a negatively adsorbing salt to have no effect, with respect to the nucleation of gas hydrate in pure water. We note that the positive or negative adsorption of a salt occurs at a much smaller scale than the thickness of a gas hydrate film which will form after the nucleation (typically several microns) in which the nucleation of gas hydrate takes place. Still, the presence of a thin aqueous layer next to the interface in which salting-out of hydrophobic guest gas takes place could slow down gas solubilization and diffusion into the underlying aqueous phase during a cooling ramp, leading to a more severe undersaturation of the guest gas than in a pure water. Conversely, the presence of a salting-in layer will facilitate gas solubilization and diffusion into the underlying aqueous phase during a cooling ramp. Morrison et al.41,42 showed that the solubility of methane and propane depends on the ions involved in the following order: NaCl < LiCl < KI. The strongest salting-out effect was observed in sodium chloride solution, and the weakest effect was recorded in potassium iodide solution. They suggested that a salting-in effect is likely to occur for large ions and/or polarizable molecules. For example, a salting-in effect may be possible in the solutions of butane (polarizable molecule) or potassium iodide (large ions), but will not appear for H2 gas (hardly polarizable molecule) in the solutions of lithium chloride (relatively small ions). They assumed that the rather complex behavior of potassium iodide was due at least in part to the dispersion and structural effects. Kiepe et al.43 reported that the strongest salting-out effect was observed in the following order: LiCl > KCl > LiBr > KBr above 1 M. The results of Morrison and Kiepe may partly explain the inhibition effect of chloride salts (a strong salting-out effect of methane in NaCl solution) and the promotion effect of iodide salts (a weak salting-out effect and/or a possible salting-in effect of methane in the solution of KI) on C1/C3 mixed gas hydrate formation investigated in our previous studies.18 Another study20 showed that the solubility of methane at 5 MPa in electrolytes at broad range of concentrations decreased as follows: KCl > NaCl > CaCl2 > MgCl2 > Na2SO4. The strong salting-out effect of MgCl2 compared to NaCl or KCl may explain our results which showed a strong inhibition effect of MgCl2 at high concentrations (above 100 mM). However,

Figure 4. Influence of electrolytes, quaternary ammonium salt and urea on C1/C3 mixed gas hydrate formation at gas pressure of 7 MPa by using HP-ALTA. The additives are classified into three groups: promoters (green), inhibitors (red), and no effect (black). Data includes results from current study (Figure 3) and previous paper*.18

By applying this rule, we conclude that chloride, bromide, and ammonium have no significant effect of their own or their effect depends on the counterion; iodide, sulfate, calcium, aluminum, potassium and lithium have promotion effects; sodium, magnesium, tetrabutylammonium, nitrate, thiocyanate, and urea have inhibition effects. These results are summarized in Figure 5. In the following sections, we discuss possible mechanisms which may account for the observed effects summarized in Figure 5.

Figure 5. Classification of ions based on their effect on C1/C3 mixed gas hydrates formation at the gas pressure of 7 MPa into three groups: promoters (green), inhibitors (red), no effect (black). It is based on the analyses of the results summarized in Figure 4. Data include results from current study (Figure 3) and previous paper.18 G

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the distance of ions from the interface may play an important role.12 When anion was a kosmotrope and cation was a chaotrope, this combination had no effect or promoted (for ammonium sulfate) the formation of C1/C3 mixed gas hydrates. According to the Hofmeister series, kosmotropic anion should not siginificantly influence the water structure in the vicinity of the interface. The presence of a strong chaotropic cation (like ammonium) may increase the solubility of gas, otherwise no significant influence on gas hydrate formation should be observed. When cation was a kosmotrope and anion was a chaotrope, this combination could result in all of the three possible outcomes: promotion, inhibition or no effect. The effects of ions in this group (chaotrope-kosmotrope) on the strucure of water and gas hydrate formation at the gas−water interface depend significantly on the strength of the chaotropic character of anions and kosmotropic character of cations. Inhibition effects were only observed (except NH4NO3) in the salt solutions of sodium (bordeline cation/weak kosmotrope) combined with a chaotropic and/or bordeline anion (Cl−, Br−, or SCN−). The inhibition effect of TBAB will be discussed later. The inhibition effect of these sodium salts on C1/C3 gas hydrate formation decreased in the order: Cl− > Br− > SCN−. This part of the results is in good agreement with the Hofmeister series. Sodium and chloride can attract water molecules at the interface stronger than bromide or thiocynate. Promotion effects were observed when a strong chaotropic anion and a weak kosmotropic cation were combined (LiI and LiBr) or when a weak chaotropic anion and a kosmotropic cation were combined (AlCl3 and CaCl2). Table 3 shows that the effectivness of anions to promote C1/ C3 mixed gas hydrate formation in monovalent salt solutions increased with their chaotropic character (I− > Br− > Cl−). In particular, iodide was a strong structure breaker that promoted gas hydrate formation (Table 3, Figure 5). The promotion effect of iodide depended on the counterion and decreased in the order of NH4I > KI > LiI > NaI. Ammonium and potassium are also strong structure breakers. Lithium was reported to decrease the hydrophobic effect and help dissolve hydrophobic molecules,49 which should be promotional. Sodium, in contrast, is a borderline ion (a weak structure maker). Thus, the strongest promotion effect was observed when both the anion and the cation were strong structure breakers. This part of our results are in a good agreement with the Hofmeister series and with some literature of experimental50 and modeling39,40 studies. Not all of our results were in good agreement with the Hofmeister series, however. The deviation was observed in multivalent salt solutions of CaCl2, MgCl2, and (NH4)2SO4. Sulfate is considered as a strong kosmotrope, thus should not significantly influence the formation of gas hydrates at the interface. All studied sulfates showed no effect on C1/C3 gas hydrate formation in dilute solutions, as expected, except for the promotion effect of (NH4)2SO4. If sulfate has no significant effect because it is a strong kosmotrope, then the promotion effect of (NH4)2SO4 must have come from ammonium. On the basis of the Hofmeister series, we may expect that ammonium has a stronger chaotropic character than potassium and yet potassium sulfate did not show any significant effect (Table 2). We note that it was reported45,51 that expected salting-out effect of sulfates decresed at low concetrations. Sulfates were

the solubility of methane does not explain the observed promotion effect of CaCl2 or the neutral effect of Na2SO4 on C1/C3 gas hydrate formation. Correlation with Hofmeister Series. Since the salting-in and salting-out properties of salts showed some effects on the nucleation of gas hydrates, we now interpret our results in terms of the Hofmeister series. We note that guest gases are hydrophobic (the surface tension of water against a gas is higher than the interfacial tension of water with an oil). Given that the salting-out of proteins is also due to the hydrophobic interaction between the hydrophobic moieties of proteins, we may expect some correlation. The Hofmeister series19,44−46 are correlated with the thermodynamic properties of ions such as entropy of hydration (tendency of the ion to accumulate in low-density water), viscosity Jones−Dole B-coefficient (results from the degree of water structuring by ions and hydration of ions by water), ionic surface tension increment and so on. The Hofmeister series is given below with respect to each ion’s ability to hydrate water molecules.19 Anions: citrate3 − > SO4 2 − > HPO4 2 − > F− > Cl− > Br − > I− > NO3− > ClO4 − > SCN− Cations: Al3 + > Mg 2 + > Ca 2 + > H+ > Na + > K+ > Rb+ > Cs+ > NH4 + > N(CH3)4

+

Structure makers (kosmotropes), which appear on the left side of the Hofemister series, strongly hydrate water molecules. Their hydration number is mostly high, and because of this tendency to remain hydrated, kosmotropes are strongly repelled from the gas−aqueous interface. In short, kosmotropes hate hydrophobic entities, are mostly excluded from the socalled low density water (LDW) and induce salting-out effect of hydrophobic molecules from aqueous solutions.21,46−48 Structure breakers (chaotropes), which appear on the right side of the Hofemister series, are mostly large, weakly hydrated ions (weak interaction with water; negative B-coefficient). Chaotropes positively adsorb to surfaces (reduces surface tension) and increase the solubility of gases (due to the tendency to accumulate in LDW and not affect the hydrogen bonding).21,46−48 On the basis of the Hofemister series, we categorized all the studied salts into four groups; chaotropes-chaotropes, kosmotropes-kosmotropes, kosmotropes-chaotropes, and chaotropeskosmotropes. We then show in Table 3 what effect each of the four group had on C1/C3 gas hydrate formation at low concentrations (below 1 M). When both anion and cation were structure breakers (chaotropes), this combination had no effect or promoted (for iodides) the formation of C1/C3 mixed gas hydrates (Table 3). The only exception is the inhibition effect of ammonium nitrate. When both anion and cation were strucutre makers (kosmotropes), this combination had no significant effect on the formation of C1/C3 gas hydrates (Table 3). According to the Hofmeister series, both cation and anion are strongly repelled from the interface. They are likely to affect the strucutre of water in the bulk. However, the interactions with water molecules in the bulk are unlikely to initiate gas hydrate formation which takes place at the aqueous−gas interface. Here H

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with surface of proteins and associated with the hydrophobic chains based.59 From the vibrational spectroscopy and molecular dynamics simulations, it was reported that the mechanism of denaturing of protein by urea was direct interaction with the protein.52 Similar conclusions were made based on the new ab initio polarizable intermolecular potential that urea did not act as a water structure breaker.63 However, we note that both hypotheses remain controversial. We observed a strong inhibition effect of urea on C1/C3 mixed gas hydrate formation at high concentrations (above 100 mM). Like kosmotropic ions, urea interacts with water molecules strongly at high concentrations and competes with guest gases for water molecules for hydrate formation. On the other hand, urea has a small positive surface tension slope with its concentration (0.22−0.35 mN/m·M) and hence is only slightly excluded from the interfacial region due to the weak negative adsorption.64 Urea may be unique in that it has both a strong salting-out (kosmotropic) property and a relatively high affinity to the interface. Recent studies showed that urea influenced the hydrogen bonds of water and enhanced CO2 gas hydrate formation and stability.65,66 Park et al.67 studied the stability of CO2 hydrate in the presence of glucose, glycine, and urea. These organic molecules affected gas hydrate formation by shifting the phase equilibrium to higher pressure and lower temperature conditions (thermodynamic inhibition effect). Amine (−NH2) and carboxylic acid (−COOH) groups are wellknown hydrate inhibitors. Glucose showed the strongest inhibition effect due to the presence of five hydroxyl groups (−OH) and the largest negative solvation energy (−24.88 kcal/ mol) among the three. Urea was reported as the weakest inhibitor among the three. It was suggested that urea contained less inhibiting amine groups and had the smallest negative solvation energy (−10.7 kcal/mol) of the three. The smallest negative solvation energy of urea suggests the lowest solubility of urea in water due to the weak association of urea molecules with water. These considerations may explain Park et al.’s results that glucose was an even more effective THI than urea. Inhibition Effects at High Concentrations. All salts are expected to behave as thermodynamic inhibitors at sufficiently high concentrations. However, we found that the strength of their thermodynamic inhibition effect varied among the studied salts. Surprisingly, the thermodynamic inhibition effect was negligible even at the highest concentration studied for some salts. We previously provided18 possible explanations to the observed phenomenon for monovalent salts due to (1) concentration dependence of the effect of ions on water activity, (2) strong salting-in effect of some salts resulting in weaker-than-expected thermodynamic inhibition effect,14 (3) transition from salting-in to salting-out possibly occurring at high concentrations. All these possilibities equally apply to the current results. Stochasticity/Width of the Probability Distributions. We observed that the probability distribution of pure water did not become as narrow as in the solutions of salts or urea. This observation is consistent with our earlier findings on monovalent salt solutions.18 A plausible explanation is that the lack of potential nucleation sites in pure water would lead to more stochastic behavior. We recently showed that the nucleation rate at a given subcooling is proportional to the local slope of the natural logarithm of a survival curve at that subcooling.33 It follows that a narrow distribution width (in the subcooling space) in a salt

also reported to increase the solubility of hydrophobic molecules (antibodies) at low concetrations.52 We note that all nitrate salts had no effect on C1/C3 mixed gas hydrate fromation, except for NH4NO3 (which inhibited). Kosmotropic sulfates with chaotropic ammonium promoted gas hydrate formation, whereas chaotropic nitrate with chaotropic ammonium hindered gas hydrate formation. If ammonium is promoting then nitrate must be strongly inhibiting. And yet all nitrate salts except for NH4NO3 had no inhibition effect. According to the Hofmeister series, MgCl2, CaCl2, and AlCl3 should not siginificantly influence nucleation of gas hydrate because of the kosmotropic character of cations (Ca2+ < Mg2+ < Al3) and neutral effect of chlorides. However, CaCl2 and AlCl3 showed promotion effects at low concentrations, whereas MgCl2 had no effect at low concetrations and a strong inhibition effect above 1 M. It was reported53 that Mg2+ increased the surface tension more strongly than Ca2+, and hence more negatively adsrobed to the interface. This may explain, at least partly, that MgCl2 had no effect at low concetrations. It can be seen that our results are not in an excellent agreement with the Hofemister series. But then, the Hofemister series was originally derived for precipitation of proteins due to the hydrophobic interaction, not for nucleation of gas hydrates. So we would not expect for the same Hofemister series to perfectly apply to the gas hydrates. We also note that previous studies showed that the precipitation of proteins by specific ions might be caused by the interactions of ions with water molecules (indirect mechanism) or interactions between ions and proteins (direct mechanism).45 If the latter were the case, then the relevance of the Homeister series to gas hydrate systems would be negligible. Effects of a Quaternary Ammonium Salt, TBAB. Most of the quaternary ammonium salts exhibited a promotion effect on gas hydrates formation.55 In contrast, our results showed a modest inhibition effect of TBAB on C1/C3 mixed gas hydrate formation at all concentrations studied (Figure 3). According to the Hofmeister series, both anion (Br−) and cation (N(C4H9)+) are relatively good structure breakers and hence should favor gas hydrate formation at the aqueous surface. However, Wen et al.54 reported that TBAB increased the solubility of methane at high temperatures and decreased it to lower than in pure water at low temperatures that were relevant to gas hydrate formation (i.e., methane was salted-out by 0.5−1 M TBAB at 268 K). This could explain the inhibition effect of TBAB we observed (Figure 3). We note that TBAB can form a semiclathrate, however, our experimental conditions favor the formation of C1/C3 clathrate hydrate. Semiclathrates of methane + TBAB aqueous system (between 0.05 and 0.45 mass fraction) can form between 281 and 291 K and at methane gas pressure up to 11 MPa.57 Effects of Urea. Urea is known as a protein denaturant at high concentrations.58 It could not be fully categorized as a structure maker or a structure breaker in aqueous solutions.58−60 The main driving force of protein denaturation by urea is undecided. Some researchers suggested that urea breaks water structure and decreases the hydrophobic effect which leads to protein denaturation.61 However, molecular dynamic simulation studies showed that urea did not break the structure of liquid water even at high concentrations. It was suggested that urea substituted for water in the hydrogen-bonded network without breaking hydrogen bonds of water.58,62 Others reported based on new solution theories that urea interacted I

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At low dosage (1 M) mostly acted as thermodynamic hydrate inhibitors, as expected. It was mostly explained due to the lower activity of water in the presence of THIs. However, some salt solutions needed much higher concentrations (>3 M) to exhibit the thermodynamic inhibition effect. Among all the studied THIs, the strongest inhibition effect on the formation of gas hydrates was found in urea (>100 mM). Therefore, urea is likely to act as an environmentally friendly thermodynamic inhibitor of gas hydrates. The quaternary ammonium salt showed an inhibition effect at all concentrations studied and hence can be considered the most consistent thermodynamic inhibitor. J

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DOI: 10.1021/acs.energyfuels.5b01391 Energy Fuels XXXX, XXX, XXX−XXX

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DOI: 10.1021/acs.energyfuels.5b01391 Energy Fuels XXXX, XXX, XXX−XXX